1. Field of the Invention
This invention relates generally to color selective polarization modulation and, more specifically, to color sequencers for colorizing imaging devices, such as displays and cameras.
2. Background of the Related Art
Full color display is generally implemented using one of four techniques: (1) spatially using color filter arrays; (2) temporally using sequential color techniques; (3) additive superposition using multiple optical paths; or (4) subtractively using stacked display panels. In spatial color systems, each full-color pixel is subdivided into at least three sub-pixels, one dedicated to each additive primary color. A color filter array (CFA), consisting of red, green and blue spectral filters, is registered to the active pixel elements of a liquid crystal display (LCD) such that the transmission level of each primary color can be locally controlled. This technique requires that the sub-pixels be sufficiently small that they are not individually resolvable by the viewer. The resulting spatial integration by the eye yields a perceived full-color image. As a result of sub-dividing, display panels used in spatial color systems require three times the number of pixels than those used in monochrome displays.
In sequential color techniques, sub-frames are displayed, with each sub-frame comprising the distribution of an additive primary color in a full-color image. By displaying the sub-frames at a sufficient rate, e.g., three-times the video rate, or 180 Hz, the eye integrates the sub-frames temporally, yielding a perceived full-color image. In this case, each pixel provides full-color because there is no spatial subdivision. In principle, a full-color pixel using a CFA provides the same brightness as a sequential pixel of the same area. However, neither makes efficient use of light, because displaying an additive primary color generally means blocking the complementary subtractive primary.
To implement a full-color display using additive superposition, as for example in a projection system, the light source is split into three optical paths, each containing the light source power in one additive primary band. Typically, dichroic beamsplitters are used to separate the three additive primary colors into three physically separate paths. One display panel is devoted to spatially modulating the optical transmission in each optical path. Subsequently, each image is additively superimposed to form a full color image. Though this technique is more hardware intensive, it is in principle three times brighter than either spatial or temporal color techniques. As such, it is the preferred technique for implementing projection systems.
In a subtractive display, three optical paths are effectively created without wavefront shearing. The term subtractive is appropriate because such systems are analogous to color film. Though all light travels the same physical path, only specific layers of the structure manipulate light in each wavelength band. In practice, a full-color display consists of a stack of three co-registered transmissive display panels, each responsible for independently determining the local transmission of one additive primary. Because there is only one physical path, each stage must be made independent of the others using wavelength selective polarization effects. Luminance modulation requires both a polarized input and an effective voltage-controlled analyzing polarizer. Thus, color independent luminance modulation is typically achieved by wavelength selectively controlling the degree of input polarization, and/or the wavelength selectivity of the analyzer. Compared to additive split-path displays, subtractive displays have unique design challenges. In order to obtain high optical throughput, panel transmission losses must be low, any passive color control elements must be low loss, and images must be efficiently relayed between panels. In direct view display systems, there are additional complications associated with color quality and parallax the when the display is viewed off-normal.
There are several subtractive display schemes disclosed in the related art. The simplest structures, such as those disclosed in U.S. Pat. Nos. 3,703,329 and 5,032,007, uses three guest host LCDs with a neutral polarizer. Each LCD panel contains a dye that acts as a color selective polarizer with in-plane projection determined by the applied voltage. In other embodiments, the function of modulation and wavelength selective polarization analysis is decoupled by combining quasi neutral LCDs with color selective polarizing films. Such polarizers can be pleochroic dye polarizers, such as those disclosed in U.S. Pat. No. 4,416,514, U.S. Pat. No. 5,122,887, and K. R. Sarma et al., SID '93 DIGEST, p. 1005, or cholesteric LC films, such as those disclosed in U.S. Pat. No. 5,686,961. Other potential color polarizer film technologies include multi-layer stretched polymer films that behave as dielectric mirrors in one linear polarization, and are isotropic in the orthogonal polarization, such as those disclosed in U.S. Pat. No. 5,612,820, and coated prismatic films, such as those disclosed in U.S. Pat. No. 5,422,756. In still other configurations, such as the configuration disclosed in U.S. Pat. No. 5,050,965, mixed-mode subtractive displays are disclosed that utilize color selective polarizers in combination with birefringence color from twisted LCD panels.
Performance of related art subtractive displays has been hampered by a number of factors. For instance, color quality and throughput are poor due to the shallow transition slope and low peak transmission of many dye polarizers. More fundamentally, the optical density of the black state is typically poor when using three subtractive filter stages.
In a subtractive mode, each additive primary is generated via the cooperative action of two stages, each blocking one additive primary. When the blocked additive primaries are adjacent primaries, there is typically an unwanted leakage. More significantly, a dense black state is obtained by subtracting all three additive primaries from white, including any interprimary light. This represents a difficult spectral management problem, because contrast ratios can plummet with even small side lobe amplitudes. Furthermore, designs that achieve acceptable contrast are frequently not robust against small fabrication tolerances, variations in modulator uniformity, and environmental changes. This is because high contrast ratio demands a high level of cooperation between stages.
Reduced side lobe levels can be obtained by increasing the overlap of each subtractive primary. However, this cannot be done without adversely affecting the color coordinates of the primary colors. While passive notch filtering can be provided to eliminate interprimary light, there is an associated insertion loss and an increase in cost. As in printing systems, a fourth “black-panel” can be inserted to improve contrast ratio, which again increases cost and complexity and reduces throughput. This “black-panel” technique is disclosed in U.S. Pat. No. 5,050,965.
Tunable filters or color shutters are well documented in the prior art. There are two classes of such filters: polarization interference filters (PIFs), and switched polarizer filters (SPFs).
PIFs have traditionally been used for spectrometry instrumentation, because they are bulky, complex to fabricate, and require calibration. Lyot PIFs consist of stand-alone filter units acting cooperatively to generate a bandpass profile. Such spectral profiles are not considered ideal for display, particularly in the blue, where adequate red blocking determines an unnecessarily narrow bandpass width. Tuning Lyot PIFs typically requires inserting analog LC devices in each stage and forming a look-up table.
The Solc PIF has the benefit that internal polarizers are eliminated, as do other filters designed using finite impulse response methods (Harris, Ahmann, and Chang). In general, however, tuning the profile of a PIF requires shifting the center wavelength of each retarder in unison. Therefore, each passive retarder must be made active with the addition of one or more LC devices. Such structures enable the generation of analog true-color, which is beneficial in spectroscopic applications. In color Display/Imaging, however, higher throughput and simpler structures are achieved using SPFs.
An SPF utilizes a digital LC switch to toggle between orthogonal polarization states. A passive component, or color polarizer, has substantially different transmission spectra associated with these polarizations. Thus, driving the LC device between states provides modulation between the two color polarizer spectra. Should the LC device be driven in an analog mode, the output typically consists of voltage-controlled mixtures of the color polarizer spectra.
Color polarizers used previously in SPFs include single (45°-oriented linear retarders on neutral polarizers (disclosed in U.S. Pat. No. 4,003,081 to Hilsum, U.S. Pat. No. 4,091,808 to Scheffer, and U.S. Pat. No. 4,232,948 to Shanks), pleochroic dye linear polarizers (disclosed in U.S. Pat. No. 4,582,396 to Bos, U.S. Pat. No. 4,416,514 to Plummer, U.S. Pat. No. 4,758,818 to Vatne, and U.S. Pat. No. 5,347,378 to Handschy), hybrid structures containing both effects (disclosed in U.S. Pat. No. 4,917,465 to Conner, and U.S. Pat. No. 5,689,317 to Miller), cholesteric liquid crystal circular polarizers (disclosed in U.S. Pat. No. 5,686,931 to Funfschilling et al.), and polarizer retarder stack (PRS) technology (disclosed in U.S. Pat. No. 5,751,384 to Sharp). Other potential color polarizer technologies include coated prismatic polarizers (disclosed in U.S. Pat. No. 5,422,756 to Weber), and multi-layer stretched polymers (disclosed in PCT Application No. WO 95/17691 to Ouderkirk).
The highest throughput SPFs currently utilize PRS technology. Because such structures use a switch external to the stack, conservation of power dictates that, assuming a neutral switch, the two spectra are substantially inverses of one another. In the absence of additional passive filtering, the spectra are highly related, with a spectral overlap at the half-maximum point. As such, retarder based SPF technology is well suited to switching between complementary colors, rather than switching between filtered and neutral outputs.
Another issue with complementary color switch technology concerns the robustness of the output. Typically, cell chrominance has a significant influence on the resulting color coordinate. While the center wavelength and uniformity of the cell are determined in fabrication, behavior is subsequently influenced by temperature, voltage, incidence angle and azimuth. For instance, when a zero twist nematic (ZTN) cell inverts the spectrum of a PRS (typically the undriven state), the half-maximum point can be shifted according to the half-wave retardance of the cell. In the driven state, the cell can be made to vanish at normal incidence, giving color polarizer limited performance. However, a significant retardence is typically experienced when viewing the filter off-normal, which can significantly influence the color coordinate. Thus, achieving a stable color coordinate using complementary switching can place impractical tolerances and, in some cases, demand unattainable performance on the cell.